Hi
First relax. You said since few days your mouth is not opening. So u need not worry since you don't take tobacco. At your age third molar erupts and that could be an issue for your limited mouth opening.
Next Steps
So go to a dentist and get X-ray and check up done to find out if that's the case. According to the X-ray or position of the molar or other reason, medication can be prescribed and treated. Not to worry.
Health Tips
U can rinse your mouth with warm salt water thrice a day and keep the ends of your jaw clean that is till last teeth. Brush properly twice a day. All the best. :) do post if this advice was helpful. Tc.